Conventional low carbon maraging steels typically contain nickel, cobalt, molybdenum, and titanium, which cooperate to provide static yield strengths that generally range from about 150 to 300 ksi. In contrast to other steels, maraging steels are primarily hardened by the formation of intermetallic precipitates, such as Ni.sub.3 Mo and Ni.sub.3 Ti. Such precipitates cause only very slight dimensional changes during hardening. In addition, maraging steels generally exhibit fracture toughness which is considerably better than that of most other high-strength steels. Consequently, maraging steels have been used in a variety of demanding applications which require relatively intricate shapes, such as fan shafts for turbine engines.
However, improvements in fatigue properties, such as fatigue strength, within maraging steels have been difficult to obtain and have typically resulted in reduced yield strength. Correspondingly, when enhanced yield strength has been sought, a reduction in fatigue properties has typically resulted. Thus, conventional maraging steel alloys generally offer either enhanced yield strength or fatigue properties, but not both. Such a result is unacceptable for demanding applications, particularly in the aerospace industry.
The fatigue life of maraging steels is dependent on crack initiation, which tend to occur at nonmetallic inclusions. Conventional maraging steels are particularly susceptible to brittle nonmetallic inclusions, such as carbides, oxides, sulfides, nitrides, carbonitrides, oxysulfides and carbosulfides, which are detrimental to strength, ductility and toughness. In the prior art, such inclusions have typically been minimized by clean melt practices Which serve to eliminate residual elements such as sulfur, oxygen, nitrogen and carbon. Other known practices employed to improve fatigue properties of maraging steels include shot peening and nitriding. Yet, existing maraging steels produced by such practices have attained relatively limited improvements in fatigue properties.
Secondary hardening steels have come into the forefront as a possible alternative to the use of maraging steels for the purpose of improving fatigue properties while retaining sufficient strength. However, to date, the desired combination of high strength coupled with adequate fatigue properties has not been achieved with the secondary hardening steels either. As with the maraging steels, it is the presence of the nonmetallic inclusions which detrimentally affect these properties.
Accordingly, it would be advantageous to provide a suitable structural steel which exhibits both enhanced yield strength and fatigue properties, and wherein nonmetallic inclusions which serve as fatigue initiation sites are substantially eliminated.